1. Field of the Invention
This invention relates to a radiation image read-out method for exposing a stimulable phosphor sheet carrying a radiation image stored thereon to stimulating rays which cause the stimulable phosphor sheet to emit light in proportion to the stored radiation energy, photoelectrically detecting the emitted light, and thereby reading out the radiation image. This invention particularly relates to a radiation image read-out method wherein, when a radiation image is read out from a stimulable phosphor sheet subjected to subdivision image recording and an image processing is conducted on a read-out image signal thus obtained, image processing conditions are adjusted to a value suitable for the respective subdivisions in the subdivision image recording. This invention also relates to an apparatus for carrying out the method.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to a radiation passing through an object to have a radiation image stored thereon, and is then scanned with stimulating rays such as a laser beam which cause the stimulable phosphor sheet to emit light in proportion to the stored radiation energy. The light emitted by the stimulable phosphor sheet when it is exposed to stimulating rays is photoelectrically detected and converted into an electric image signal, which is processed as desired to reproduce a visible image on a recording medium such as a photographic film or on a display device such as a cathode ray tube (CRT).
The radiation image recording and reproducing system using a stimulable phosphor sheet is advantageous over conventional radiography in that the image can be recorded over a very wide range (latitude) of radiation exposure and further in that the electric signal used for reproducing the visible image can be freely processed to improve the image quality for viewing, particularly for diagnostic purposes. More specifically, since the amount of light emitted upon stimulation after the radiation energy is stored on the stimulable phosphor sheet varies over a wide range in proportion to the amount of said stored energy, it is possible to obtain an image having desirable density regardless of the amount of exposure of the stimulable phosphor sheet to the radiation, by reading out the emitted light with an appropriate read-out gain and converting it into an electric signal to reproduce a visible image on a recording medium or a display device. The electric signal may further be processed as desired to obtain a radiation image suitable for viewing, particularly for diagnostic purposes. This is very advantageous in practical use.
As mentioned above, in the radiation image recording and reproducing system using a stimulable phosphor sheet, compensation for deviation of the level of the radiation energy stored on the stimulable phosphor sheet from a desired level can easily be carried out by adjusting the read-out gain to an appropriate value when photoelectrically reading out the light emitted by the stimulable phosphor sheet upon stimulation thereof. Therefore, the quality of the reproduced radiation image is not adversely affected by a fluctuation in radiation dose due to fluctuating tube voltage or MAS value of the radiation source, variation in the sensitivity of the stimulable phosphor sheet or the photodetector, changes in radiation dose resulting from differences in the condition of the object, or differences in radiation transmittance of the object, and the like. Also, it is possible to obtain a desirable radiation image even when the radiation dose to the object is low. Further, it is possible to obtain a radiation image having a high image quality of high contrast, high sharpness and low noise, and the like, by converting the light emitted by the stimulable phosphor sheet into an electric signal, and processing the electric signal as desired.
However, in order to eliminate various influences caused by variations in radiographic exposure conditions and/or to obtain a radiation image having a high image quality or a high diagnostic efficiency and accuracy, it is necessary to investigate such image input conditions of the radiation image stored on the stimulable phosphor sheet as, for example, the level of radiation dose used for image recording, or the image input pattern which is determined by the portion of the object (e.g. the chest or the abdomen of the human body) or the image recording method used, such as plain image recording or contrasted image recording, before reproducing the radiation image to a visible image, and then to adjust the read-out gain to an appropriate value or the process the electric signal appropriately based on the detected image input conditions or the image input pattern. The image input conditions and the image input pattern will hereinafter be simply referred to as the image input information when they are referred to generically. It is also necessary to determine the scale factor to optimize the resolution in accordance with the contract of the image input pattern, and to adjust image processing conditions to appropriate values in the case where image processing such as gradation processing is conducted on the read-out image signal.
Investigation of the image input information may be conducted prior to the visible image reproduction by use of the method as disclosed in Japanese Unexamined Patent Publication No. 58(1983)-67240. In the disclosed method, a read-out operation for detecting the image input information of a radiation image stored on a stimulable phosphor sheet (hereinafter referred to as the preliminary read-out) is conducted in advance by use of stimulating rays having stimulation energy of a level lower than the level of the stimulation energy of stimulating rays used in a read-out operation for obtaining a visible image for viewing, particularly for diagnostic purposes (hereinafter referred to as the final read-out), and thereafter the final read-out is carried out. In the final read-out, the read-out gain and/or the scale factor is adjusted to an appropriate value, and/or an appropriate image processing is conducted, on the basis of the image input information obtained by the preliminary read-out.
As described above, the level of the stimulating rays used in the preliminary read-out is lower than the level of the stimulating rays used in the final read-out. That is, the effective energy of the stimulating rays which the stimulable phosphor sheet receives per unit area in the preliminary read-out should be lower than the effective energy of the stimulating rays used in the final read-out. In order to make the level of the stimulating rays used in the preliminary read-out lower than the level of the stimulating rays in the final read-out, the output of the stimulating ray source such as a laser beam source may be decreased in the preliminary read-out, or the stimulating rays emitted by the stimulating ray source may be attenuated by an ND filter, an AOM, or the like, positioned in the optical path. Alternatively, a stimulating ray source for the preliminary read-out may be positioned independently of the stimulating ray source for the final read-out, and the output of the former may be made lower than the output of the latter. Or, the beam diameter of the stimulating rays may be increased, the scanning speed of the stimulating rays may be increased, or the moving speed of the stimulable phosphor sheet may be increased in the preliminary read-out.
In the aforesaid method, since the image input conditions and the image input pattern of a radiation image stored on the stimulable phosphor sheet can be investigated in advance, it is possible to obtain a radiation image having an improved image quality, particularly a high diagnostic efficiency and accuracy, by adjusting the read-out gain and the scale factor in the manner most suitable for the image input pattern on the basis of the detected image input information without using a read-out system having a wide dynamic range.
When a radiation image is recorded on the stimulable phosphor sheet, subdivision image recording is often carried out. In subdivision image recording, the recording region on the stimulable phosphor sheet is divided into a plurality of subdivisions, and the respective subdivisions are exposed to a radiation for image recording. Subdivision image recording is economical since, for example, when an image of a small object portion is recorded on a large stimulable phosphor sheet, images of a plurality of object portions may be recorded on a single stimulable phosphor sheet. Also, the radiation image recording and read-out processing speed becomes high.
However, in the case where the image processing conditions in the final read-out are adjusted based on the image input information obtained by the preliminary read-out when a radiation image stored on a stimulable phosphor sheet subjected to the subdivision image recording is read out, the image processing conditions become unsuitable for the radiation image. Specifically, preliminary read-out has heretofore been conducted over the entire recording region on the stimulable phosphor sheet or over a part thereof slightly smaller than the overall recording region. Therefore, the image processing conditions adjusted based on the preliminary read-out image signal obtained by the preliminary read-out become suitable for the whole radiation image stored on the stimulable phosphor sheet. However, in subdivision image recording, radiation doses to the respective subdivisions may differ greatly from each other. In such a case, the image processing conditions adjusted as described above may become unsuitable for the radiation images stored at some subdivisions. For example, in the case where the recording region on the stimulable phosphor sheet is divided into two subdivisions and radiation image recording is conducted by exposing one of the two subdivision to a markedly high radiation dose and exposing the other thereof to a markedly low radiation dose, the image processing conditions adjusted based on the preliminary read-out as mentioned above become suitable for an image of the middle level between the two radiation images actually stored at the subdivisions, and become unsuitable for both of the two radiation images.
The aforesaid problem is caused not only by a difference in radiation dose but also by a difference in the image recording portion of the object, the image recording method, or the like between the subdivisions of the recording region.
The aforesaid problem arises also when instead of conducting the preliminary read-out, the image input information is ascertained at the time of the final read-out and the image processing conditions are adjusted based on the image input information thus ascertained.